1. Field of the Invention
The invention relates to a method of reducing a spatial energy spread within an electron beam, the electron beam being widened by means of an energy-dispersive member.
The invention also relates to an electron beam apparatus suitable for using such a method.
2. Description of the Related Art
An electron beam apparatus of this kind is known from the abstract in English of JP-A 63-231852. Energy spread occurs within a beam from an electron source due to initial energy spread of the emitted electrons and energy spread due to statistic interactions of electrons in the beam (Boersch effect). An electron beam apparatus described therein comprises, subsequent to the condensor lens, an energy-dispersive member, notably a Wien filter, in combination with a slit, extending transversely of the dispersion direction of the dispersive member, in order to reduce the energy spread of the electron beam.
The operation of the filter is based on a predetermined relation between the strength of the electric field E, the strength of the magnetic field B and the acceleration potential of the electrons. In a Wien filter the electric field and the magnetic field cross one another at right angles and both fields are directed perpendicularly to the direction of propagation of the electrons. The acceleration potential required to ensure that electrons having a given velocity v are not deflected follows from the above relation. Electrons incident on the filter with an energy which deviates from the energy of the electrons of the velocity v are distributed over a spectrum. The slit subsequent to the filter transmits exclusively the non-deflected electrons, thus reducing the energy spread of the electron beam.
It is a drawback that due to the use of a slit a comparatively large part of the beam current is intercepted, i.e. only the electrons situated within the desired energy window are transmitted. Moreover, providing a slit in a high voltage environment is very critical and hence comparatively intricate and expensive.
It is an object of the invention to provide a method and an electron beam apparatus in which substantially the entire beam current can be used and in which said drawback is mitigated.
According to a first aspect of the invention, the method is characterized in that the energy-dispersive member is arranged in the vicinity of an electron source generating the electron beam, the electron beam being widened by said member in a dispersion direction with a dispersivity which is greater than the diameter of the electron source, a number of pixels of local energy spread being selected in an image plane of an object.
Local energy spread is to be understood to mean the energy spread of the incident beam, measured at an infinitely small point of the object.
Because all electrons within an electron beam do not have the same energy, due to initial energy spread of the emitted electrons and energy spread due to statistic interactions of the electrons in the beam, the beam will be widened in a dispersion direction after passing the dispersive member. As a result, the local energy spread of the electron beam, being the energy spread of the electrons per pixel, will be reduced relative to the original energy spread. The reduction may be substantial when the ratio of the dispersion to the source diameter is sufficiently great.
Due to the energy dispersion, the mean local energy measured across the object changes in the dispersion direction. The ultimate energy selection is realised by implicit selection of a number of pixels in the image of the object. This means that electrons of different pixels are separately detected in the imaging mode, so that a slit in the electron beam path can be dispensed with. The local energy spread is thus substantially reduced.
A version of the method in accordance with the invention is characterized in that the selection of the number of pixels is performed by tilting the object.
Because the mean energy of the widened electron beam incident on the object varies in the dispersion direction, defocusing will occur across the object in that direction in the TEM as well as in STEM images. A fully focused line can be obtained by tilting the object. Usually, however, such defocusing will be within the depth of focus so that it can be ignored.
A version of the method in accordance with the invention in which transmission analysis of the object takes place is characterized in that an image processing procedure is applied in which the energy spread is used as a parameter.
When the transmission electron microscopy utilizes an image processing procedure for image reconstruction, the energy spread of the beam and the variation of the energy across the object can be taken into account in the transfer function used.
Another version of the method in accordance with the invention in which transmission analysis of the object takes place is characterized in that an image part of specific local energy spread is selected by means of a selective member arranged in an image plane of the object.
Thus, in the image plane of the object a part of the image is selected by means of a selective member arranged in the image plane. This member may be a slit or other diaphragm, for example a so-called "Selected Area" diaphragm. This selection can also be performed by means of the detector itself, for example by means of a CCD detector arranged in the desired position in the image plane. Another possibility in this respect is the use of the entrance diaphragm of an imaging electron-energy-loss spectrometer. The advantage over the state of the art resides in the fact that selection does not take place at the high-voltage level, but only in the image plane of the object. As a result, inter alia the full beam current can be used to irradiate the object.
In accordance with a second aspect of the invention, the method is characterized in that the member is arranged in the vicinity of an electron source generating the electron beam, the object being step-wise scanned by the widened electron beam in a direction in which the object is focused and which extends substantially perpendicularly to the dispersion direction, the magnitude of the steps being determined by a desired resolution.
Widening of an electron beam on the basis of energy dispersion is particularly attractive. When the spot of the electron beam is widened, a larger part of the object can be irradiated per scanning measurement. The desired resolution can be achieved as a function of the magnitude of the steps in the direction perpendicular to the dispersion direction. An electron beam apparatus in accordance with the invention which is suitable to execute a method as claimed comprises an electron source, an electron-optical system, a specimen holder and an energy-dispersive member.
A further embodiment of the electron beam apparatus in accordance with the invention is characterized in that the electron beam apparatus comprises an object tilting system.
A further embodiment of the electron beam apparatus in accordance with the invention is characterized in that the electron beam apparatus comprises an image processing system.
A further embodiment of the electron beam apparatus in accordance with the invention is characterized in that the electron beam apparatus comprises a diaphragm which is arranged in an imaging plane of the object.
Another embodiment of an electron beam apparatus in accordance with the invention is suitable to carry out a method as claimed and comprises an electron source, an electron-optical system, a specimen holder, an energy-dispersive member and a scanning unit.
A further embodiment of the electron beam apparatus in accordance with the invention is characterized in that the electron beam apparatus comprises an imaging analysis system, it being possible to measure a spectrum in a direction extending substantially perpendicularly to a direction in which the object can be focused.
Thus, in the dispersion direction spectral information is produced per pixel situated within the widened spot, whereas similar information can be obtained for other pixels in the scanning direction, substantially perpendicular to the dispersion direction, when the widened spot is shifted. Thus, the spectral information is measured line-wise per image. Because an object part to be analysed is irradiated only by a part of the electron beam of local energy spread during such an analysis, a resolution can be achieved which is substantially higher than when irradiation takes place by means of an electron beam without energy-dispersion.
A further embodiment of the electron beam apparatus in accordance with the invention is characterized in that the electron beam apparatus comprises a correction member which includes a multi-pole element for correction of chromatic aberration.
The resolution in the length direction of the spot of the electron beam is determined by the electron probe. The resolution in the width direction of the spot of the electron beam, i.e. dispersion direction, is determined by the chromatic aberration of the objective lens.
In order to prevent the occurrence of electrons of deviating energy, due to the chromatic aberration, in an area of local energy spread, a correction element can be provided in the principal plane of the objective lens. This correction element preferably comprises a combined magnetic and electrostatic quadrupole for correction of the chromatic aberration in the dispersion direction.
A preferred embodiment of the electron beam apparatus in accordance with the invention is characterized in that the energy-dispervive member is arranged in a given position in a beam path between the electron source and a conjugate object plane, said position corresponding to an acceleration potential of less than 20 kV.
A conjugate object plane is to be understood to mean any plane equivalent to the object plane.
The dispersive member is preferably arranged in a position in which the acceleration potential is sufficiently low, so that the electrons have a comparatively low energy and can hence be comparatively readily influenced. Between the electron source and the energy-selective filter there may also be provided, if desired, a lens or other electron-optical elements.
A further embodiment of the electron beam apparatus in accordance with the invention is characterized in that the energy-dispersive member is a Wien filter.
Another feasible embodiment of the energy-dispersive member is formed by a .OMEGA.-filter or an electrostatic filter. Both filters are known per se.
A Wien filter offers the advantage that it is a "direct-vision" member so that practical problems such as in respect of alignment are minimized. A Wien filter is known in the art from inter alia British Patent Specification GB 1,364,930.
A further embodiment of the electron beam apparatus in accordance with the invention is characterized in that the electron source is a field emission source.
The intrinsic source diameter of a field emission source is much smaller than the source diameter of a conventional electron source. Consequently, for a comparable energy spread of a conventional source and a field emission source, after dispersion the spot produced by the field emission source will exhibit a substantially greater length-to-width ratio.